Halliday & Matthiessen (1999: 266-7):
Relators show the least organisation of any, since they are experienced only indirectly in the form of logical relations between other configurations; they share some of the systematic features of minor processes, but other than that they display only the contrast between the two relative statuses they assign to these configurations, as being equal or unequal (paratactic/ hypotactic, in the grammar) — a then x / x after a // b so y / y because b &c. Thus a relator can be metaphorically reconstrued into any other category:
— whereas a process can be reconstrued only as a participant (quality or thing), and a quality only as a thing.